The present invention refers to a rear axle for motorcycles of the type employing a Cardan shaft for transmitting motive power from the motor to the rear driving wheel.
Although far less popular than chain drive systems, Cardan-shaft drives for transmitting motive power in motorcycles are a subject covered by a number of patent publications, most of which provide however for the use of two swing arms connecting the rear wheel to the frame of the motorcycle, albeit with a single damping unit—see for instance DE-A-698 258, CH-A-318 696 and DE-A-1 055 981.
A number of further examples of rear axles for motorcycles are known in the art, in which a Cardan drive shaft is coupled to a single swing arm, although the state of art that is considered as being the most relevant one to the purposes of the present invention is represented by the two patent publications that are more closely examined below.
In both such patents, the housing accommodating the bevel gears used to transmit motion to the driving wheel is not rigidly linked to the swing arm, but is rather hinged thereto in such a manner as to be floating. Furthermore, it is guided by means of a torque bar connected to the frame, with the result that the moment generated by the contact forces of the rear wheel contacting the ground is partially opposed, i.e. compensated for by said torque bar. This again results in both a smaller extension of the suspension when the motorcycle is accelerating and a smaller compression of the same suspension when the motorcycle is braking, i.e. slowing down. Accordingly, the motorcycle is enabled to behave in a more agreeable manner.
In particular, the patent publication U.S. Pat. No. 4,434,868 discloses a rear axle which is formed by a four-bar linkage, the main component parts of which are the swinging suspension arm, which encloses the Cardan drive shaft with its two universal joints, and the torque or link arm, which bears the whole weight of the motorcycle since the end portions thereof are hinged on to the frame and to a downwards extending appendix of the housing accommodating the bevel gears, respectively. The main drawback connected with such a kind of construction lies in the fact that, in the case of said torque arm and/or the pin connecting the latter to said housing breaking down (an event that is really not so unlikely to occur considering the vicinity of these parts to the ground), the motorcycle becomes unusable since it is no longer capable of sustaining itself or even collapses.
In U.S. Pat. No. 5,067,580, the swing arm of the rear axle carries the lower end portion of a damper and connects the frame with a pivot whose axis does not coincide with the axis of rotation of the rear wheel; in turn, the torque or link arm is arranged above the swing arm and extends parallel thereto, as well as co-planar with the Cardan drive shaft. Such a construction, apart from being inherently expensive owing to the relatively large number of component parts involved, is exposed to obvious risks deriving from the fact that the drive shaft in its entirety, i.e. including its two universal joints, is unguarded and consequently is exposed to bumps against obstacles that may be encountered during running. Moreover the possibility exists for the lower hem of garments (trousers or skirts) worn by the persons using the motorcycle to be caught in the drive shaft.